In recent years, fiber-reinforced composite materials using reinforcing fibers such as carbon fiber and aramid fiber, owing to the high specific strength and specific elastic modulus thereof, have been used as structural materials of aircraft and motor vehicles, sporting applications such as tennis rackets, golf shafts, fishing rods and the like, general industrial uses, etc. Methods for producing the fiber-reinforced composite materials include a method of laminating a plurality of prepregs obtained as a sheet-like intermediate material including reinforcing fibers impregnated with an uncured matrix resin and then thermally curing the laminated prepregs, a resin transfer molding method of pouring a liquid resin into the reinforcing fibers arranged in a mold and then thermally curing the resin, etc.
Among these production methods, a method of using a prepreg has an advantage that a high performance fiber-reinforced composite material is easily obtained since orientation of reinforcing fibers can be strictly controlled and the degree of freedom of design of a laminate constitution is high. As the matrix resin used in the prepreg, from the viewpoint of heat resistance and productivity, thermosetting resins are mainly used. Particularly, epoxy resins are suitably used from the viewpoint of adhesion between a resin and reinforcing fibers, dimensional stability and mechanical properties such as strength and rigidity of the resulting composite material.
Among the epoxy resins, amine type epoxy resins which provide a cured product having a small epoxy equivalent and a high crosslinking density have been used as a matrix resin for a fiber-reinforced composite material for aerospace applications requiring excellent strength properties and durable stability. While the amine type epoxy resin provides a resin cured product having a high elastic modulus and high heat resistance, it tends to provide a resin cured product having its small ability to deform and low toughness.
Thus, methods of mixing a rubber component or a thermoplastic resin respectively excellent in toughness to form a with the epoxy resin have been tried as a method of improving the toughness of the epoxy resin. However, there have been problems that these methods tend to cause a reduction of elastic modulus, deterioration of heat resistance, deterioration of processability due to thickening, or quality loss such as void formation.
For this situation is proposed in recent years a method in which a fine phase separation structure is stably formed in a curing process of an epoxy resin by mixing a block copolymer such as a copolymer composed of a styrene-butadiene-methyl methacrylate or a block copolymer composed of butadiene-methyl methacrylate to significantly improve the toughness of an epoxy resin (Patent Document 1, Patent Document 2). However, in these techniques, there is still a tendency incapable of imparting adequate toughness to the epoxy resin since the amount of the block copolymer to be mixed has to be reduced to avoid the adverse effect on processability. A cured product of such an epoxy resin exhibits high water absorption in a high-temperature and high-humidity environment and has a problem that strength properties as the fiber-reinforced composite material are inadequate in a high-temperature and high-humidity environment.
As another method of improving the toughness of the epoxy resin, a method of using an epoxy resin in combination with a rigid epoxy resin which can provide strength properties and heat resistance while suppressing a crosslinking density, has been tried (Patent Documents 3 and 4). For example, Patent Document 4 discloses that it is possible to attain the toughness and the heat resistance by the combined use of the amine type epoxy resin and a fluorene type epoxy resin. However, in such a method, the elastic modulus of the resin cured product and the strength properties as the fiber-reinforced composite material may be still inadequate.
In recent years, the epoxy resin itself is being improved, an epoxy resin capable of significantly reducing the crosslinking density while ensuring heat resistance at a level of an aircraft material, has been developed. For example, Patent Document 5 discloses that extremely high heat resistance and an extremely high elastic modulus can be achieved simultaneously by curing a fluorene type epoxy resin having an introduced condensed polycyclic group with a phenol novolak resin. However, also in this case, the resulting resin cured product has been low in elongation and brittle and has not led to a significant improvement of toughness. Further, the amount of the epoxy resin to be mixed and the combination of the epoxy resin with another component have not been referred to at all, and it can be said that there are no findings concerning physical properties of an epoxy resin composition obtained by use of these epoxy resins.